1. Field of the Invention
The present invention relates to an optical head device for recording and/or reproducing an information signal on and/or from an information recording medium, an aberration correction method of an aberration correcting element used in such an optical head device and an optical information processing apparatus provided with such an optical head device.
2. Description of the Background Art
High-density and high-capacity optical information recording media include optical discs such as DVDs and Blu-Ray discs (hereinafter, abbreviated as “BDs”). Such optical discs are recently rapidly growing popular as recording media for recording images, music and computer data.
As optical disc capacities increase, the wavelengths of light sources for optical head devices are becoming shorter and the apertures of objective lens are becoming larger. Since the dispersion of an optical material such as a lens element is very high in a short wavelength region, the refractive index of the optical material largely changes upon a slight change in the wavelength of a beam. Accordingly, it has been necessary to consider a correction of chromatic aberration in optical head devices of recent years.
Here, since a laser power is generally higher at the time of recording information than at the time of reproducing information in an optical head device, a phenomenon of momentarily varying a central wavelength by several nm due to an output change upon the switch from the reproduction to the recording might occur in some cases. A defocus error that occurs due to this phenomenon can be eliminated by focusing an objective lens. Unless the chromatic aberration of the objective lens is not corrected, problems such as a recording failure caused by the defocus error occur during a period of several nsec until the objective lens is focused. Since the longitudinal chromatic aberration of the objective lens increases as the wavelength of the light source of a beam passing through the objective lens becomes shorter, deterioration in wavefront aberration resulting from this phenomenon tends to increase as the light source wavelength becomes shorter. For the above reason, it can be said that the longitudinal chromatic aberration needs to be corrected particularly in an optical head device using a blue violet semiconductor laser as a light source.
A diffraction element utilizing a diffraction action is known as an element for correcting the longitudinal chromatic aberration of an objective lens with a simple construction. Optical head devices using a blue violet semiconductor laser and including a diffraction element for correcting the longitudinal chromatic aberration of an objective lens are disclosed in Japanese Unexamined Patent Publication Nos. 2001-256672 (prior art 1), No. 2001-108894 (prior art 2) and No. 2002-082280 (prior art 3) described below.
The optical head device disclosed in the above prior art 1 is designed to correct the longitudinal chromatic aberration of an objective lens by a diffraction element arranged in a parallel beam between a blue-violet semiconductor laser light source and the objective lens. The optical head device disclosed in the above prior art 2 is designed to correct the longitudinal chromatic aberration of an objective lens by the action of a diffraction structure on an optical surface of a collimator lens for converting a divergent beam from a blue-violet semiconductor laser light source into a parallel beam and introducing the parallel beam to an objective lens. Further, the optical head device disclosed in the above prior art 3 is designed to correct the longitudinal chromatic aberration of an objective lens by the action of a diffraction structure on an optical surface of an expander lens arranged in a parallel beam between a blue-violet semiconductor laser light source and the objective lens.
In the conventional optical head devices, a beam emerging from a chromatic aberration correcting element toward the objective lens becomes a convergent beam if the wavelength of the semiconductor laser changes in such a direction as to become longer than the designed wavelength of an optical system for the optical head device by the action of the diffraction structure. Further, if the wavelength of the semiconductor laser changes in such a direction as to become shorter than the designed wavelength of the optical system for the optical head device, a beam emerging from the chromatic aberration correcting element toward the objective lens becomes a divergent beam. The longitudinal chromatic aberration of the objective lens can be corrected utilizing this characteristic. If the degree of divergence of the beam propagating toward the objective lens changes in this way as the wavelength of the semiconductor laser changes, the magnification of the objective lens changes, wherefore spherical aberration occurs.
Semiconductor lasers used as light sources in optical head devices have wavelength differences of about ±10 nm from laser to laser by manufacturing errors. If a semiconductor laser whose wavelength is deviated from the designed wavelength of the optical system for the optical head device is used in the above optical head devices, the spherical aberration occurring as the magnification of the objective lens changes is eliminated. Thus, it is necessary to initially adjust the position of the collimator lens and that of the semiconductor laser, which increases the production cost of the optical head device.
Particularly, the above problem tends to become more obvious by using a single objective lens having a large numerical aperture as a general method for reducing the cost of the optical head device and making the construction of the optical head device compact. With the single lens, the spherical aberration when the wavelength of an incident light changes (hereinafter, called “spherical chromatic aberration”) increases in proportion to the fourth power of the numerical aperture. Thus, it is necessary to correct the spherical chromatic aberration remaining in the objective lens itself in addition to the spherical aberration change resulting from the change in the magnification of the objective lens during the initial adjustments of the position of the collimator lens and that of the semiconductor laser. In order to realize the use of the single objective lens having a large numerical aperture, it is preferable to use a highly refractive material to ensure a margin for optical axis deviations of optical surfaces.
However, since a highly refractive material is generally highly dispersed, an amount of the longitudinal chromatic aberration to be corrected by the chromatic aberration correcting element tends to increase. Thus, in order to correct the longitudinal chromatic aberration of the objective lens made of such a highly refractive material, the degree of divergence of a beam propagating from the chromatic aberration correcting element toward the objective lens, which results from the wavelength change of the semiconductor laser, needs to be set large. As a result, a change in the magnification of the objective lens increases in the case of using a semiconductor laser whose wavelength is deviated from the designed wavelength of the optical head device. Thus, the spherical aberration increases as the magnification of the objective lens changes, and amounts of initial adjustments of the position of the collimator lens and that of the semiconductor laser increase.
As a solution to such a problem, the spherical aberration occurring as the magnification of the above objective lens changes can be canceled by designing the chromatic aberration correcting element such that the spherical aberration changes if the wavelength of the semiconductor laser changes.
However, there is a likelihood that if the optical axis is deviated by the tracking drive for the objective lens, nonnegligible magnification chromatic aberration occurs by a wavelength change caused by the power variation from the reproduction to the recording and, therefore, no good tracking characteristic can be obtained, which might cause a recording or reproduction failure.
Further, the longitudinal chromatic aberration of only the objective lens has been corrected by the chromatic aberration correcting element in the conventional optical head devices. For example, CDs and DVDs are made up of at most two information recording layers, and it is sufficient for a laser beam from the light source to have a low power if the number of information recording layers is small. Thus, the longitudinal chromatic aberration by the optical system other than the objective lens was negligible amounts.
Contrary to this, with the advent of BDs in recent years, optical discs tend to have higher speeds and more layers. By the speeding up and higher stratification of optical discs, laser beams from light sources are required to have higher powers. In such a case, the wavelength of the light source varies by several nm or more at the time of the switch from the reproduction to the recording. Thus, the amount of the longitudinal chromatic aberration by the optical system other than the objective lens becomes nonnegligible, making it difficult to suppress the longitudinal chromatic aberration of the entire optical system from the light source to the objective lens.